1. Field of the Invention
This invention relates to a process for machine dishwashing in institutional dishwashing machines in which a detergent and another active substance supporting the detergent in its effect are added to the wash liquor of the dishwashing machine. The invention also relates to an arrangement for carrying out the process which comprises a detergent feed system and a feed system for another active substance with an associated pump and pump control system.
An institutional dishwashing machine normally contains several tanks arranged in tandem from which wash liquor is sprayed against the crockery passing through the dishwashing machine. The tanks are arranged in the form of a cascade, the wash liquor passing successively through the tanks from the crockery exit or outlet end to the crockery entrance or inlet end, so that the degree of soiling of the wash liquor increases from the outlet end to the inlet end. Fresh water is introduced into the dishwashing machines at the outlet end. The quantity of detergent required is introduced into at least one washing tank also known as the feed tank. The detergent is normally added automatically in dependence upon the conductivity or the pH value of the wash liquor or, where liquid detergent or powder-form detergent already dissolved in water is added, even by means of a timed feed pump.
In normal machine dishwashing, starch deposits which build up on the crockery often cannot be prevented and existing starch deposits cannot be removed with the detergents used in practice in the usual in-use concentrations. Accordingly, crockery affected by starch deposits is subjected to so-called thorough cleaning at certain time intervals. In the thorough cleaning process, a distinctly above-average concentration of detergent is established in the wash liquor. Another alternative is to spray a highly concentrated alkaline detergent onto the crockery in the course of a routine dishwashing cycle. In addition, manual tank cleaning is also possible
2. Discussion of Related Art
It is known from DE-OS 17 28 093, which relates to dishwashing in domestic dishwashing machines, that a rinse aid may be added together with amylase to the rinsing water in order to remove starch deposits on the crockery. If desired, protease or lipase may be added to the rinse aid in addition to the amylase.
In addition, DE-AS 12 85 087 describes a machine dishwashing process in which an alkaline detergent is added to the dishwashing machine in the main wash cycle while an enzyme-containing, more particularly amylase-containing, rinse aid is introduced in the final rinse cycle and optionally in the prewash cycle. The object of this is to degrade starch formed on the crockery in the final-rinse cycle and optionally in the prewash cycle. However, it is specifically pointed out that the enzyme-containing rinse aid cannot be added in the main wash cycle because the alkalinity of the detergent would immediately destroy the ferments.
3. Description of the Invention
In the Journal Fette, Seifen, Anstrichmittel, 73 (1971), No. 7, page 464, left-hand column, third-to-last paragraph, it is summarily pointed out that enzyme-containing detergents cannot be used in institutional dishwashing machines on account of the long contact times required. In view of the brief contact time in which the crockery comes into contact with the wash liquor in institutional dishwashing machines, it would not appear possible to the expert that starch deposits on crockery can be prevented or degraded with enzyme-containing detergents in institutional dishwashing machines.
A corresponding process and an arrangement for carrying out this process are known from DE-A-39 20 728. In this known machine dishwashing process for institutional dishwashing machines, active oxygen is introduced into the feed or washing tank of the dishwashing machine in addition to the detergent as the other active substance supporting the detergent in its effect. To maintain the concentration of oxygen in the washing tank in the event of interruptions in the wash cycle, more active oxygen is introduced into the washing tank during the interruptions.
The object of the present invention was to provide a solution which would permanently suppress the formation of starch deposits on the crockery during machine dishwashing in institutional dishwashing machines.
In a process of the type mentioned at the beginning, the solution provided by the invention is characterized in that a low-alkali detergent, more particularly based on phosphate or nitrilotriacetic acid or salts thereof (NTA), is added as the detergent while a detergency booster containing an enzyme, preferably a carbohydrate-degrading enzyme, more particularly an amylase-containing detergency booster, is added as the additional active substance.
In an arrangement of the type mentioned at the beginning for carrying out the process, the solution provided by the invention is characterized by a feed system for an enzyme-containing detergency booster which is separate from the detergent feed system and which comprises an operational regime for maintenance feeding during interruptions in or stoppage phases of the operation of the dishwashing machine and/or feed intervals of the detergent feed system and/or an operational regime for surge feeding after interruptions in or stoppage phases of the operation of the dishwashing machine and/or feed intervals of the detergent feed system.
It has surprisingly been found that a low-alkali detergent in typical concentrations in conjunction with an enzyme-containing detergency booster leads to excellent removal of and inhibition of starch deposits on crockery, even over the brief contact times of 10 to 180 seconds typical of institutional dishwashing machines. Compared with known dishwashing processes which use a highly alkaline detergent or a highly concentrated alkaline wash liquor, the process according to the invention is distinguished by a considerable improvement in operational and applicational safety. There is no longer any risk of injury to operating personnel by highly alkaline detergent or wash liquor.
The enzyme-containing detergency booster may be introducedxe2x80x94in the same way as typical detergentsxe2x80x94either into at least one feed or washing tank of the dishwashing machine or even into the liquid flowing through the final rinse pipe and/or the spray system of the institutional dishwashing machine and added in this way to the dishwashing machine. The detergency booster may contain as enzyme amylase, lipase, protease or other enzymes, more particularly carbohydrate-degrading enzymes, either individually or in the form of suitable mixtures. If desired, the low-alkali detergent may optionally contain other complexing agents than those mentioned.
In one embodiment of the invention, a concentration of 0.5 to 15 g/l of low-alkali detergent and a concentration of 0.05 to 2 g/l of detergency booster may be established in the wash liquor.
In another advantageous embodiment of the invention, a low-alkali detergent is introduced in the in-use concentration with a pH value of 7 to 11 and preferably in the range from 9.1 to 10.8.
In another particularly advantageous embodiment, the detergency booster introduced contains around 0.01 to 0.6% by weight and preferably 0.45 to 0.55% by weight of enzyme, particularly amylase, and 10 to 25% by weight and preferably 15 to 20% by weight of propylene glycol, more particularly 1,2-propylene glycol, and a corresponding quantity of water.
In another embodiment of the invention, the enzyme-containing detergency booster is added to the wash liquor at the same time as or after the low-alkali detergent during regular operation of the dishwashing machine at typical detergent concentrations of 0.5 to 8 g/l in the wash liquor and/or during periodic thorough cleaning at an increased concentration of detergent in the wash liquor of 3 to 15 g/l.
It is known that enzymes, such as amylase, lipase or protease, are not stable in the wash liquor of institutional dishwashing machines. After they have been introduced into the washing tank of an institutional dishwashing machine, enzyme-containing detergents or detergency boosters lose their effect relatively quickly. In the event of interruptions in or stoppage phases of the operation of the dishwashing machine and/or intervals in the feed of the detergent or detergency booster, enzyme degradation or enzyme decomposition (consumption) occurs to such an extent that the enzyme content of the wash liquor often falls at a rate of around 40 to 60% per hour. Depending on the degree of consumption, however, the enzyme content may fall to far more than half, for example even after an interruption of only 30 minutes in the operation of the machine. In order to ensure that a sufficient concentration of enzyme to obtain a satisfactory cleaning result is present in the wash liquor after an interruption in or stoppage phase of the operation of the dishwashing machine and/or an interval in the feed of the detergent or detergency booster, another embodiment of the invention is characterized in that, in the event of interruptions in or stoppage phases of the operation of the dishwashing machine and/or intervals in the feed of the detergent, the enzyme-containing detergency booster undergoing the enzyme degradation or enzyme decomposition (consumption) under the washing conditions prevailing in an institutional dishwashing machine is added to the wash liquor in a quantity which equalizes the degradation or decomposition (consumption) of enzyme during the particular interruption or stoppage phase and/or feed interval so that, after the particular interruption or stoppage phase and/or feed interval, the operation of the dishwashing machine is continued with substantially the same concentration of enzyme in the wash liquor as was present before the particular interruption or stoppage phase and/or feed interval. This ensures that a sufficiently high concentration of enzyme to obtain the required cleaning result (preventing the buildup or removal of starch deposits on the crockery) is present in the wash liquor during each active cleaning phase of the wash cycle in an institutional dishwashing machine.
Enzyme may already be incorporated in the liquid or powder-form low-alkali detergent. More particularly, a solid enzyme carrier, for example an amylase carrier, may be incorporated in a powder-form low-alkali detergent. This enzyme-containing detergent is then used in combination with the enzyme-containing detergency booster. For this particular case, another embodiment of the invention is characterized in that a low-alkali detergent, more particularly a powder-form low-alkali detergent, containing sufficient quantities of an enzyme for immediate washing is introduced into the wash liquor during detergent feed periods and in that the enzyme-containing detergency booster is introduced into the wash liquor immediately after or during the interruptions or stoppage phases and/or the detergent feed intervals to maintain the concentration of enzyme, i.e. to equalize the degradation or decomposition (consumption) of enzyme. In this case, therefore, the enzyme-containing detergency booster is only introduced into the wash liquor during or immediately after the detergent feed intervals and/or the interruptions in or stoppage phases of the operation of the dishwashing machine. There is no introduction of the enzyme-containing detergency booster during those periods of the active wash cycle in which the detergent is introduced into the washing tank of the dishwashing machine.
However, it is also possible to use an enzyme-free, liquid or powder-form low-alkali detergent in combination with an enzyme-containing detergency booster. For this particular case, another embodiment of the invention is characterized in that an enzyme-free, more particularly liquid, low-alkali detergent and at the same timexe2x80x94commensurate with the consumption of detergentxe2x80x94the enzyme-containing detergency booster are introduced into the wash liquor during detergent feed times and in that, immediately after or during the interruptions or stoppage phases and/or the detergent feed intervals, the enzyme-containing detergency booster is introduced into the wash liquor to maintain the enzyme concentration, i.e. to equalize the degradation or decomposition (consumption) of enzyme. In cases where an enzyme-free detergent is used in combination with an enzyme-containing detergency booster (in contrast to the above-described case where an enzyme-containing detergent is combined with an enzyme-containing detergency booster), enzyme-containing detergency booster is introduced into the at least one feed or washing tank of the institutional dishwashing machine at the same time as the detergent, even during the feed of detergent or the detergent feed times.
According to the invention, there are two alternatives for the introduction or subsequent feeding of the enzyme-containing detergency booster. On the one hand, the enzyme-containing detergency booster may be introduced or subsequently added during the interruptions in or stoppage phases of the operation of the dishwashing machine and/or the detergent feed intervals; on the other hand, the enzyme-containing detergency booster may be introduced or subsequently added immediately after the interruptions in or stoppage phases of the operation of the dishwashing machine and/or the detergent feed intervals.
In the first alternative, the concentration of enzyme in the wash liquor during the particular interruption or stoppage phase and/or the detergent feed intervals is maintained by maintenance feeding of the enzyme-containing detergency booster. In one advantageous embodiment of the invention, the maintenance feeding takes place in individual feed strokes.
In this first alternative, therefore, the invention is concerned primarily with maintenance feeding of the enzyme-containing detergency booster during the stoppage phases of the dishwashing machine between two successive wash phases or during the intervals between two detergent feed times. The effect of maintenance feeding is that new enzyme-containing detergency booster enters the at least one feed or washing tank commensurately with the degradation or decomposition (consumption) of enzyme. Accordingly, the washing tank or rather the wash liquor of the dishwashing machine is always kept in readiness for a new wash phase. Wash liquor containing sufficient enzymes or a sufficiently high enzyme concentration is immediately available at the beginning of each wash phase. Where detergent, more particularly powder-form detergent, containing a sufficient quantity of enzyme for immediate washing is used, the enzyme-containing detergency booster is only introduced into the at least one washing tank during the stoppage phases or detergent feed intervals commensurately with the degradation of enzyme. Where an enzyme-free detergent, more particularly a liquid detergent, is used, the enzyme-containing detergency booster is introduced in addition to maintenance feeding during the detergent feed times. Accordingly, where the enzyme-free detergent is used, enzyme is introduced into the washing tank or into the wash liquor during the wash phases commensurately with the consumption of detergent while enzyme-containing detergency booster is introduced into the washing tank or into the wash liquor during the stoppage phases or the detergent feed intervals commensurately with the degradation or decomposition of enzyme. Parallel feeding provides for the use of enzyme-containing detergency booster, for example amylase solution, which cannot be formulated with the usual enzyme-free alkaline detergents, more particularly low-alkali detergents. Liquid or powder-form detergent is introduced during the rinse phases as a function of the measured conductivity or the measured pH value or on a timed basis only during certain feed times. If the corresponding intervals or rather detergent feed intervals become so long that enzyme is degraded in a quantity detrimental to the performance of the next rinse phase, it is possible in accordance with the invention subsequently to introduce enzyme-containing detergency booster, again in the form of maintenance feeding, commensurately with the degradation of enzyme during intervals in the parallel feeding regime occurring in the wash phase.
In another embodiment of the invention relating to the maintenance feeding regime, the enzyme input rate of the maintenance feeding regime is optimized on the basis of enzyme activity determination. Once the decomposition rate of the enzymes is known, it may be sufficient to add enzyme-containing detergency booster at certain time intervals. In this connection, another embodiment of the invention is characterized in that maintenance feeding of the enzyme-containing detergency booster is commenced after the enzyme content of the wash liquor has fallen by around 20%.
In another embodiment of the invention, the other of the two alternatives mentioned above is characterized in that the particular interruption or stoppage phase and/or feed interval is immediately followed by surge feeding in which enzyme-containing detergency booster is added to the wash liquor in a quantity corresponding to the degradation or decomposition (consumption) of enzyme which has taken place during the particular interruption or stoppage phase and/or feed interval. In this embodiment, therefore, the interruption in or stoppage phase of the operation of the dishwashing machine and/or the detergent feed interval is followed by surge feeding of the enzyme-containing detergency booster, the quantity of enzyme-containing detergency booster added during surge feeding being adapted to the duration of the interruption and gauged in such a way that it corresponds to the degradation or decomposition (consumption) of enzyme which has taken place during the interruption. Accordingly, no enzyme-containing detergency booster is added to the at least one feed or washing tank or to the wash liquor of the dishwashing machine during the interruption in or stoppage phase of the operation of the machine and/or the detergent feed interval. The feed of enzyme-containing detergency booster only takes place after the particular interruption or interval over a relatively short period so that the quantity of enzyme which has been consumed during the particular interruption or feed interval is replaced. The quantity of detergency booster added during surge feeding is gauged as a function of the duration of the interruption and/or feed interval and as a function of the trend of the enzyme decomposition process which can be described by a mathematical function, for example an e-function. This ensures that the enzyme consumed is replaced fairly accurately without any significant underdosage or overdosage.
In another preferred embodiment of the present invention, exact feeding geared to demand is achieved by taking into account the fact that the consumption of enzyme as a function of time essentially follows an exponential function, the concentration decreasing exponentially from a starting concentration. Accordingly, satisfactory surge feeding after an interval t must follow the complementary function of the course of the enzyme consumption process.
As in the maintenance feeding regime, it is possible in the surge feeding regime to add the enzyme-containing detergency booster at the same time as the detergent during the detergent feed times in cases where an enzyme-free detergent is used.
The arrangement according to the invention for carrying out the process according to the invention is basically characterized by a feed system for the enzyme-containing detergency booster which is separate from the detergent feed system and which comprises an operational regime for maintenance feeding and/or an operational regime for surge feeding. The detergent feed system may be designed in the usual way, for example as a feed pump in the case of a liquid detergent or as a freshwater or liquor dispensing system in the case of a powder detergent. According to the invention, therefore, the only addition is essentially the feed system for the enzyme-containing detergency booster which is separate or separated from the detergent feed system. The feed system for the enzyme-containing detergency booster has either an operational regime for maintenance feeding or an operational regime for surge feeding and is provided with the technical means required for the particular operational regime. However, besides this embodiment where the feed system for the detergency booster only has either technical means for the maintenance feeding regime or technical means for the surge feeding regime, the feed system for the detergency booster may also be provided with technical means for both regimes so that the particular operational regime required can be freely selected by the machine operator. Accordingly, this arrangement provides for maintenance feeding during the interruptions in or stoppage phases of the operation of the dishwashing machine and/or the detergent feed intervals or for surge feeding immediately after interruptions in or stoppage phases of the operation of the dishwashing machine and/or detergent feed intervals, so that any reduction in enzyme activity occurring during that time through the degradation or decomposition of enzyme is compensated. Such subsequent or extra feeds are necessary in cases where the supply of detergent to the institutional dishwashing machine is regulated by a control system which does not take the enzyme content of the wash liquor into consideration. In contrast to the enzyme content, the concentration of detergent in the wash liquor remains substantially constant during interruptions or stoppage phases of the dishwashing machine. Without the subsequent introduction of enzyme, it would otherwise only be detergent that would normally be introduced into the at least one feed or washing tank or into the wash liquor of the dishwashing machine commensurate with the supply of fresh water at the beginning of a new wash cycle. This would result in a deficient enzyme content and hence in an unsatisfactory cleaning result, at least in regard to starch deposits. This problem is overcome by the arrangement according to the invention.
In one embodiment of the arrangement for carrying out the surge feeding regime, the arrangement comprises a counter to which interval pulses are regularly delivered during the interruptions. These interval pulses do not increase the count linearly, but instead in a step-by-step function complementary to the decomposition of enzyme, more particularly an e-function. Finally, the count asymptotically approaches a predetermined end value at a minimum counting rate. After the interruption or stoppage phase and restarting of the machine, the counter is counted linearly downwards at a constant rate from the count reached until the count reaches zero. During the downward counting process, the downward counting pulses activate a pump which triggers the surge feeding regime. Accordingly, the duration of the surge feeding regime is determined by the count reached during the interruption. The duration of the surge feeding regime is shorter by orders of magnitude than the duration of the interruptions in question. Accordingly, in a matter of seconds to minutes, the surge feeding regime compensates the consumption of active substance over several tens of minutes to hours. Accordingly, the count is a measure of the duration of the surge feeding regime. This duration is always far shorter than the duration of intervals, so that the surge feeding of the enzyme-containing detergency booster takes place at a far higher rate than the degradation or decomposition (consumption) of the enzyme in the wash liquor. In this way, the consumption of enzyme over several tens of minutes is compensated after only a very short time of a few minutes. This form of surge feeding takes place not only immediately after interruptions in or stoppage phases of the operation of the dishwashing machine, but if desired even immediately after detergent feed intervals.
In the context of the invention, the term xe2x80x9ccounterxe2x80x9d is meant to be interpreted in its broadest sense. It encompasses any counting device which counts the interval pulses delivered to it in accordance with a predetermined function and, hence, also provides for non-linear counting processes, for example those following an e-function. The variation in the time interval between the pulses may be used to accelerate (compress) or delay (extend) the counting process in terms of time. The interval pulses may be separated by constant time intervals and may be multiplied by a factor corresponding to an e-function in terms of time. On the other hand, the interval pulses may also be delivered to the counter with different time intervals. Finally, the counter may also incorporate an adding stage which, with each interval pulse, increases the count by an amount varying as a function of time.
In another embodiment of the invention which is applicable both to arrangements with the maintenace feeding regime and to arrangements with the surge feeding regime, the feed system for the enzyme-containing detergency booster has an additional operational regime for parallel feeding which operates in parallel with the detergent feed system when the detergent feed system is switched on. In this embodiment, both enzyme-containing and enzyme-free detergent can be used in the arrangement, the parallel feeding regime providing for the introduction of enzyme-containing detergency booster where an enzyme-free detergent is used. The parallel feeding regime and the maintenance feeding regime or the surge feeding regime essentially differ only in the amount of enzyme-containing detergency booster which is delivered to the feed tank or to the wash liquor per unit of time.
In another advantageous embodiment of the invention, a frequency-controlled flow inducer or a diaphragm pump is used to deliver the enzyme-containing detergency booster. A pump of this type may be operated with a number of strokes per unit of time corresponding to the decomposition of enzyme. Accordingly, relatively few pump strokes are sufficient for the maintenance feeding regime whereas a far greater number of pump strokes is required for the surge feeding regime and/or for the parallel feeding regime, if any, where an enzyme-free detergent is used. Accordingly, it has proved to be useful in practice if the associated frequency control system of the pump has a first control range for the maintenance feeding regime and a second control rangexe2x80x94with a far greater output by comparison with the first control rangexe2x80x94for the wash phase of the dishwashing machine when the detergent feed system is switched on.